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Topic: HD600 - Asus Xonar U7 vs. SB Omni Surround (Read 24626 times) previous topic - next topic
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HD600 - Asus Xonar U7 vs. SB Omni Surround

Reply #25 –
PS:
Quote from: Yours truly on
Setups 1) and 3) show copious amounts of what looks to be ground loop noise spoiling the results. Not sure why the Omni is better here - maybe someone bothered to implement an (electronically) balanced input?

I missed the obvious, namely that the Omni is the one recording. It thus has the smallest ground loop of them all by far in loopback, and most importantly said ground loop does not extend over USB power ground into the notebook and back! I would advise retesting the others while using their own inputs. Easier than making a ground-loop-breaking cable or even a balanced input circuit, anyway.

Using one of these combo mic/line inputs is not ideal (lowish input impedance) but tends to yield quite decent results IME. You just need to disable preamplification (it probably defaults to something like +20 dB), then the input ought to accept 1-1.4 Vrms - or maybe things can be switched altogether (jack retasking). Mic bias voltage tends to be  ~3 V and applied through resistors of 1-2.2 kOhms, so it's not particularly dangerous.

Once that is sorted, I'd be interested to know how well the Omni does with 44.1 in playback.

HD600 - Asus Xonar U7 vs. SB Omni Surround

Reply #26 –
Is there any reason why you need 44.1kHz? It does 48kHz very well so why not just set that in the Windows 7/8 playback device options and forget about it? Foobar2000 has a quality resampler if you are concerned with the windows mixer's SRC quality.

HD600 - Asus Xonar U7 vs. SB Omni Surround

Reply #27 –
I did a test of U7's own loopback and it changed everything. Thanks for the suggestion, stephan_g. I have uploaded the results in the Uploads thread. The result is excellent.

Is there any way how I can mark the thread as SOLVED? I can't edit my initial post.

HD600 - Asus Xonar U7 vs. SB Omni Surround

Reply #28 –
That's much better. You can clearly see that the opamp used to drive the output seems to break a bit of a sweat - the dominant odd-order distortion is telling (though -90 dB is nothing much to worry about yet, especially since levels appear to be near maximum), as well as the increase towards higher frequencies in the swept sine test. You would see it even better at 96 kHz - distortion actually does not drop above 6 kHz and change, it's just that the dominant 3rd harmonic is being attenuated by the anti-alias filter then.

Apparently the Xonar U7 uses a TI / NatSemi LME49726MA opamp - a type with substantial output current that is suitable for lowish operating voltages like +5 V. At just 0.7 mA per amplifier typical (1.4 mA for a stereo pair), substantial amounts of crossover distortion are not at all surprising. For illustration, the classic NJM4556A (for higher voltages) consumes 4 mA per side - even if only half of that is output stage quiescent current, it's still about 4 times as much as I'd expect for the NatSemi part, possibly even more. The NatSemi part can't make up for it in GBW either, as it actually is slightly slower.

If we are seeing dominant 3rd into an easy 300 ohm load, I would expect results to be less than pretty with insensitive 32-50 ohm cans. It might have been a better idea to put up with the notoriously diva-like AD8397 or somesuch instead.

I would assume that the Omni fares better in this respect, at least if it uses the same MAX97220 found in other modern-day Creative cards. My Audigy FX shows a slight increase in swept IMD towards 10 kHz when driving ~1 Vrms into a HD580 at the front output in 24/96, but otherwise seems hardly bothered. Things still are under -80 dB at this point. With some extrapolation, the U7 would seem to barely hit -60 dB.

HD600 - Asus Xonar U7 vs. SB Omni Surround

Reply #29 –
About the 44.1kHz playback... The Omni does not even have any 44.1 options in the Windows Playback or Recording device configuration. There are only 4 options - 16/24 bit 48/96kHz. I guess it is not possible to test native 44.1kHz without Windows resampling it to 48 or 96 kHz (since the device does not have that capability).

HD600 - Asus Xonar U7 vs. SB Omni Surround

Reply #30 –
I updated the uploads thread with some low-impedance load measurements. It seems the amplifier in Omni is somewhat better (less affected by the load) than in U7.

Unfortunately, both devices seem to further boost (+1dB) the already overblown base of Sennheiser HD518. The L/R separation suffers from low impedance equally on both devices.

HD600 - Asus Xonar U7 vs. SB Omni Surround

Reply #31 –
Quote from: stephan_g on
That's much better. You can clearly see that the opamp used to drive the output seems to break a bit of a sweat - the dominant odd-order distortion is telling (though -90 dB is nothing much to worry about yet, especially since levels appear to be near maximum), as well as the increase towards higher frequencies in the swept sine test. You would see it even better at 96 kHz - distortion actually does not drop above 6 kHz and change, it's just that the dominant 3rd harmonic is being attenuated by the anti-alias filter then.

Apparently the Xonar U7 uses a TI / NatSemi LME49726MA opamp - a type with substantial output current that is suitable for lowish operating voltages like +5 V. At just 0.7 mA per amplifier typical (1.4 mA for a stereo pair), substantial amounts of crossover distortion are not at all surprising. For illustration, the classic NJM4556A (for higher voltages) consumes 4 mA per side - even if only half of that is output stage quiescent current, it's still about 4 times as much as I'd expect for the NatSemi part, possibly even more. The NatSemi part can't make up for it in GBW either, as it actually is slightly slower.

If we are seeing dominant 3rd into an easy 300 ohm load, I would expect results to be less than pretty with insensitive 32-50 ohm cans. It might have been a better idea to put up with the notoriously diva-like AD8397 or somesuch instead.

I would assume that the Omni fares better in this respect, at least if it uses the same MAX97220 found in other modern-day Creative cards. My Audigy FX shows a slight increase in swept IMD towards 10 kHz when driving ~1 Vrms into a HD580 at the front output in 24/96, but otherwise seems hardly bothered. Things still are under -80 dB at this point. With some extrapolation, the U7 would seem to barely hit -60 dB.


What do you see that you consider to be reliable evidence of crossover distortion?

HD600 - Asus Xonar U7 vs. SB Omni Surround

Reply #32 –
A considerable increase in HD towards higher frequencies when driving anything resembling a load is very suspect, for one thing. If you extrapolate towards 10 kHz, it's far from where I'd expect in line out (quasi-unloaded) operation in a competent soundcard.

The very low quiescent current draw makes things even more suspect. Besides, -90 dBr @ 1 kHz is a far cry from what the LME49726 datasheet claims for a buffer driving 3.5 Vrms into 10 kOhms (0.00008%, i.e. <-120 dBr).

Thanks for asking though (even if the fullquote would not have been necessary) - otherwise I wouldn't have realized that the dominant odd-order distortion profile is likely to indicate a push-pull type opamp circuit, most likely push-pull AB (given the shoestring current budget). Unfortunately I'm a bit out of the loop when it comes to literature showing such circuitry in CMOS (you don't get to within 4 mV of the rails with BJT output). Some bipolar implementations mentioned in the classic Tietze-Schenk include OP467, LT1352 and LM7171; Groner also mentions LT1354-LT1363; his OP467 measurement results show the dominant odd-order distortion very clearly, especially in the output linearity test with 600 ohm loading.

Qualitatively similar results are also seen in the Clip+, another MOS IC affair. You can set output stage bias levels for its headphone driver (3 amps, 1 being a virtual ground driver) in software, IIRC the standard value is 0.5 mA per amp. Upping this to a nominal 0.75 mA yields a very modest but measurable reduction of high-frequency harmonics when driving loads (BTDT). I don't think they're selling the headphone driver section separately somewhere.

HD600 - Asus Xonar U7 vs. SB Omni Surround

Reply #33 –
Quote from: stephan_g on
A considerable increase in HD towards higher frequencies when driving anything resembling a load is very suspect, for one thing. If you extrapolate towards 10 kHz, it's far from where I'd expect in line out (quasi-unloaded) operation in a competent soundcard.

The very low quiescent current draw makes things even more suspect. Besides, -90 dBr @ 1 kHz is a far cry from what the LME49726 datasheet claims for a buffer driving 3.5 Vrms into 10 kOhms (0.00008%, i.e. <-120 dBr).

Thanks for asking though (even if the fullquote would not have been necessary) - otherwise I wouldn't have realized that the dominant odd-order distortion profile is likely to indicate a push-pull type opamp circuit, most likely push-pull AB (given the shoestring current budget). Unfortunately I'm a bit out of the loop when it comes to literature showing such circuitry in CMOS (you don't get to within 4 mV of the rails with BJT output). Some bipolar implementations mentioned in the classic Tietze-Schenk include OP467, LT1352 and LM7171; Groner also mentions LT1354-LT1363; his OP467 measurement results show the dominant odd-order distortion very clearly, especially in the output linearity test with 600 ohm loading.


I would never expect the analog output of a battery-powered IC-based headphone amp to be anything but push-pull.  If for no other reason than power efficiency. But p-p also supressed even order distoriton.

I always remember that when we look at measurements of these devices we can't separate the converter performance from the analog buffer performance without opening the box, and perhaps not even then.

The 100% evidence of crossover distortion is a discontinuity in the devices ouptut voltage near Output Voltage= Zero. This is usually verified by playing a small signal and enlarging it so that such a thing could be seen on a Voltage versus Time plot.  Sometimes partially notching out the test signal can improve its visibility, if it exists.

Quote
Qualitatively similar results are also seen in the Clip+, another MOS IC affair. You can set output stage bias levels for its headphone driver (3 amps, 1 being a virtual ground driver) in software, IIRC the standard value is 0.5 mA per amp. Upping this to a nominal 0.75 mA yields a very modest but measurable reduction of high-frequency harmonics when driving loads (BTDT). I don't think they're selling the headphone driver section separately somewhere.


I can't see where the author of that report called out the Clip for having crossover distortion.  I have a Clip and a couple of Fuzes and the necessary testing hardware and software, and if it weren't that my day job is making heavy demands right now, I'd do some specific tests.  Whatever the cause of the distortion is, it is still like 70 dB down at worst, and lots of luck detecting that in a proper listening test!  That all said I listen to my Fuze via a NX-1 buffer for reasons of dynamic range and relatively insensitive IEMs, so I don't have a dog in this fight. ;-)

HD600 - Asus Xonar U7 vs. SB Omni Surround

Reply #34 –
Mostly CDs picked up at Estate Sales. Rip to Lame 256K because MP3 works just about everywhere and 256K is plenty good for these 568 year old ears.

HD600 - Asus Xonar U7 vs. SB Omni Surround

Reply #35 –
Cross over distortion would show up in those DAC linearity tests right?  He doesn't show the first harmonic in that test, but presumably if there was a huge peak there, he would have commented on it.

HD600 - Asus Xonar U7 vs. SB Omni Surround

Reply #36 –
Quote from: saratoga on
Cross over distortion would show up in those DAC linearity tests right?  He doesn't show the first harmonic in that test, but presumably if there was a huge peak there, he would have commented on it.


Crossover distortion as well as other low level nonlinearities show up in the Dynamic Range test which is based on a test tone that is 60 dB below FS.  Delta Sigma DACs are the rule and they are inherently linear. In practice they are not perfectly linear but their nonlinearity comes primarily from their analog side.

When someone shows a THD test with all (both even and odd) harmonics comparably represented but uniformly decreasing with order, the wave to think about is a Sawtooth wave being superimposed on the signal.  If you notch out the test signal, that's the basis of what is left over. 

People with CEP or Audition can generate these at will by generating a Triangle/Sawtooth wave with a Flavor of 0% or 100%.  The FFT analysis will show every harmonic with decreasing amplitudes, reminiscent of the odd harmonics in a Square Wave.
 

HD600 - Asus Xonar U7 vs. SB Omni Surround

Reply #37 –
Quote from: Arnold B. Krueger on
I would never expect the analog output of a battery-powered IC-based headphone amp to be anything but push-pull.  If for no other reason than power efficiency. But p-p also supressed even order distoriton.

Especially if it extends right up to the input stage, which is a point that possibly I should have been stressing a bit more. I would not be surprised to see a fully Class AB ("power on demand") circuit like what I mentioned.

Quote from: saratoga on
Cross over distortion would show up in those DAC linearity tests right?  He doesn't show the first harmonic in that test, but presumably if there was a huge peak there, he would have commented on it.

We're talking analog here, not DAC nonlinearity. Some of the latter is there in the Clip+, but the measured deviation (-90 dBFS tone at -91.6dBFS) computes to 0.05% of distortion re: -40 dBFS. So even in the quiet parts of a fairly dynamic classical recording it would still be about 60 dB down - noise level is expected to be higher than that in many cases. IOW, nothing I'd lose much sleep over, particularly since volume control is analog and follows after the DAC, easing up on DAC requirements by orders of magnitude (which saves heaps of power).

Contrary to popular belief, analog crossover distortion in properly-designed AB amplifiers actually decreases when reducing amplitude. At some point it tends to reach sort of a plateau, and eventually proceeds to drop further as Class A operation is reached. Actually even the "plateau" is a numerical thing, as you'll see the dropoff towards higher-order harmonics varying quite considerably - a hearing-weighted measure of harmonic distortion would be anything but flat.

If you look at the Clip+ 400 mV into 16 ohm measurements, you can see that the decay in higher-order harmonics is very slow. Halve the amplitude, and you would see them decay much more quickly, until at some point the distortion profile becomes benign.

Now if you've got a real discontinuity / dead zone there (Class B or C, depending on whose definition you follow), then things do in fact tend to get nasty. This tends to result in amplifiers whose distortion profile at full output isn't so bad, but once the output amplitude approaches the order of dead zone width, distortion becomes pretty awful (preceding circuitry desperately jumps between the ends of the dead zone in an attempt to maintain linear output, taxing its slew rate). Hence why some people emphasize "first watt" performance so much, and for good reason - 1 W tends to be good for >80 dB SPL after all.

Several years ago I simulated a headphone amplifier circuit using a 5532-ish opamp plus underbiased Class B Darlington booster within the feedback loop (~1.8 V bias, so maybe 0.4-0.6 V missing), and obtained:
(10 kHz THD, 300 ohm load - worse at lower impedance)
6 Vrms: 0.15%
0.6 Vrms: 0.7%
0.06 Vrms: 0.6%
0.006 Vrms: 0.02%
Yikes. (And from what I remember, mid-level distortion spectra weren't exactly looking pretty either.) The real deal seemed to measure better than that, but I still didn't like its sound much when driving 100 ohm cans.

Similar bias-less circuitry has also been looked at in practice in a small speaker amp. Unsurprisingly, opamp slew rate becomes a major factor. (Yes, the TL071 should theoretically be slewing faster than the NE5534, but its high internal output impedance means that it's losing a lot of voltage internally, especially around the deadband region where buffer transistor beta is low. This type is notorious for its poor output driving abilities, worse than the '741's, while a '5534 is pretty rugged in that regard.)